The catalytic reaction of SO.sub.2 and oxygen to form SO.sub.3 in contact-catalysis systems is now effected with the aid of catalysts that contain V.sub.2 O.sub.5 and alkali sulfate as active substances.
Such catalysts are required to effect a high conversion of SO.sub.2 to SO.sub.3 at a low reaction temperature and to have a low susceptibility to higher temperatures and to have a high mechanical strength. These properties depend to a large degree on the method by which the catalyst is produced.
French patent No. 538,139 describes a process in which catalyst-support bodies are impregnated with a pure solution of vanadium in sulfuric acid. The resulting catalyst effected only a conversion below 95% and about 1000 liters of catalyst were required per metric ton of H.sub.2 SO.sub.4 produced per day. These poor properties are probably the reason why an acid impregnation has not been used thereafter.
From B. Waeser "Die Schwefelsaurefabrikation", 1961, Verlag Friedrich Vieweg und Sohn, Braunschweig, Germany, pages 258 to 258, and from German patent document 12 81 407, German patent document 15 42 177, Austrian patent 116,361 and U.S. Pat. No. 1,941,426 it is known to impregnate support bodies with alkaline or neutral solutions which contain V.sub.2 O.sub.5 and alkali compounds.
Impregnation with alkali involves the risk of a chemical attack on the SiO.sub.2 content of the substrate bodies, such that the mechanical strength and the internal surface area are decreased.
Where a neutral impregnation is effected, the impregntion and drying steps must be repeated because the active substances are of low solubility. As a result, the exact adjustment of the active-substance content is difficult and production is expensive.
From B. Waeser "Die Schwefelsaurefabrikation", 1961, it is also known to form a paste of kieselguhr and solutions of the active substances and to make the catalyst bodies from said paste.
By that process it is difficult to make bodies which are uniform in composition and the danger of the formation of defective catalysts is high.